U.S. Pat. No. 5,104,380 describes a generally pen-like syringe which incorporates a dose metering device provided by a cap or “dose knob” which is rotatable with respect to the pen body to a position related to the dose of medication to be injected. Rotation loads (by twisting) a coil spring which is prevented from unwinding by cooperating teeth. The user disengages the ratchet teeth by means of a sliding trigger coupled to the outside of the pen body, resulting in the unwinding of the spring. This in turn causes rotation of a drive gear which is coupled to a syringe plunger via a quick pitch screw thread. Rotation of the gear results in axial movement of the plunger through the pen body. The plunger acts on a “bung” of a medicament containing cartridge, driving medicament from the cartridge as it travels. The structure of the syringe of U.S. Pat. No. 5,104,380 is such that, once a dose has been set by a user dialing the dose knob to a given position, the user cannot reduce the setting by winding the dose knob in the reverse direction. If the dose has been overset, the user must activate the trigger to expel the set dose, and redial the correct dose.
WO02/053214 describes a syringe comprising a similar dose setting mechanism. The design described in this document is claimed to allow the dose knob to be rotated in the reverse direction to allow an overset dose to be reduced.
According to a first aspect of the present invention there is provided medication delivery apparatus comprising:                a housing for receiving a medication containing member;        a drive member mounted within the housing for engaging with the medication containing member and moveable axially within the housing;        a spring contained within the housing and coupled to the drive member;        a dose setting knob coupled to said spring, and rotatably coupled to the housing such that rotation of the knob relative to the housing in a first direction results in compression or twisting of the spring;        a user actuable trigger for releasing the spring to push the drive member through the housing; and        a user actuable button coupled to said housing for axial motion relative thereto, said button being coupled to said spring to cause the spring to unwind or expand without causing any substantial movement of said drive member.        
Preferably, actuation of said button causes said spring to unwind or expand in discrete steps with each press of the button
In an embodiment of the present invention, the apparatus further comprises a drive shaft extending through the housing, the drive shaft being coupled via a ratchet mechanism to a drive element, and the drive element being coupled to said drive member, whereby rotation of the drive element gives rise to axial movement of the drive member. Said dose setting knob is coupled to the drive shaft for rotation therewith when the user actuable button is in a released position, and is decoupled from the drive shaft when the button is in a depressed position whereupon the dose knob is able to rotate relative to the drive shaft.
Said user actuable trigger is configured to release said drive element for rotation by the spring when the trigger is actuated, and to secure the drive element when the trigger is in its resting state.
Said ratchet mechanism preferably comprises two complimentary sets of teeth, a first set formed on an end of the drive shaft and a second formed on the drive element. With the trigger in its resting state, the teeth of the drive shaft are able to ride over the teeth of the drive unit to allow rotation of the drive shaft relative to the drive element when the dose knob is rotated in a dose setting direction.
Preferably, said user actuable button is coupled to the drive shaft for rotation therewith. The drive shaft comprises means for locking the drive shaft to the dose knob, which means is released when the button is depressed by a user. The locking means may comprise one or more teeth for engaging with teeth of a rack provided around a surface of the dose knob, which teeth are free to flex inwardly when the button is depressed. The button may further comprise means for inducing rotation of the dose knob in a dose reducing direction when the button is depressed, and means for defining the dose reducing steps. These means may be provided by an indexing finger depending from the user actuable button and which engages a rack formed around a surface of the dose knob.
In order to assist a user who requires more than one type of medicament, the apparatus may further comprise an annular member that is removably attachable to an outer surface of the main body, the annular member being used to identify a medicament.
According to a second aspect of the present invention there is provided medication delivery apparatus for receiving a medication containing member, the apparatus comprising:                spring means for storing a force which when released causes ejection of the medication from a received medication containing member;        a dose knob which when rotated in a first direction moves the spring to a force storing position, the dose setting knob providing a plurality of discrete setting positions corresponding to respective doses;        a user actuable trigger for releasing said stored force to eject medication from said medication containing member; and        a user actuable button arranged to reduce a stored force in the spring without causing ejection of medication from said medication containing member.        
Preferably, the apparatus comprises a drive means for engaging a medication containing chamber, and a drive shaft coupled between the drive means and said spring means. In a resting state, the dose knob is coupled to the drive shaft for rotation therewith. When the button is depressed, the dose knob is at least temporarily uncoupled from the drive shaft to allow the dose knob to rotate relative to the drive shaft.
According to a third aspect of the present invention there is provided a method of setting a dose of medication to be ejected from a medication containing member received within a dose delivery apparatus, the method comprising:                rotating a dose knob of the apparatus in a first direction to one of a plurality of discrete positions corresponding to respective doses; and        changing the set dose to a reduced dose by depressing a button one or more times.        
According to a fourth aspect of the present invention there is provided medication delivery apparatus comprising:                a housing for receiving a medication containing member;        a drive member mounted within the housing for engaging with the medication containing member and moveable axially within the housing;        a spring contained within the housing and coupled to the drive member;        a dose setting knob coupled to said spring, and rotatably coupled to the housing such that rotation of the knob relative to the housing in a first direction results in compression or twisting of the spring;        spring retaining means comprising a toothed rack coupled to one of the housing and the dose knob and at least one spring mounted tooth coupled to the other of the housing and the dose knob, the or each tooth engaging the toothed rack to prevent a force stored on the spring from moving the spring to release the force, whilst allowing a user to rotate the dose knob in a second, reverse direction, to reduce a set dose; and        a user actuable trigger for releasing the spring to push the drive member through the housing.        
In a preferred embodiment of the invention, the or each spring mounted tooth has a one piece moulded construction, with a tooth element mounted on an end of a moulded spring.
Preferably, the apparatus comprises an elongate drive shaft coupling said spring to said drive member. An end of the drive shaft is coupled to the dose knob for rotation therewith. The other end of the drive shaft provides part of said spring retaining means, either said at least one sprung tooth or said toothed rack. The other of said at least one sprung tooth and said toothed rack is provided on a drive element which is coupled to the housing via said user actuable trigger. The drive element is in turn coupled to the drive member. Actuation of the trigger releases the drive element to rotate within the housing under the action of the spring via the drive shaft. Rotation of the drive element produces axial movement of the drive member through the housing.
According to a fifth aspect of the present invention there is provided medication delivery apparatus comprising:                a housing for receiving a medication containing member;        a drive member mounted within the housing for engaging with the medication containing member and moveable axially within the housing;        a torsion spring contained within the housing and coupled to the drive member;        a dose setting knob coupled to said spring, and rotatably coupled to the housing such that rotation of the knob relative to the housing in a first direction results in loading of the spring;        a user actuable trigger for releasing the spring to push the drive member through the housing; and        a rotatably mounted visual indicator for indicating the dose to be delivered, the rotatably mounted visual indicator being rotatable over at least one complete revolution.        
According to a sixth aspect of the present invention, there is provided medication delivery apparatus comprising:                a housing for receiving a medication containing member;        a drive member mounted within the housing for engaging with the medication containing member and moveable axially within the housing;        a torsion spring contained within the housing and coupled to the drive member;        a dose setting knob coupled to said spring, and rotatably coupled to the housing such that rotation of the knob relative to the housing in a first direction results in loading of the spring;        a dial cam configured to co-operate with a dial key, such that upon rotation of the dose setting knob, the dial cam and the dial key co-operate to strain or release the torsion spring, depending upon the direction of rotation of the dose setting knob.        